1. Field of the Invention
The present invention relates to an induction heating apparatus. More specifically, the present invention relates to an induction heating apparatus wherein a heating coil and a switching device connected thereto in series are included and a current flowing through the heating coil, that is, a power is controlled by turning on or off this switching device.
2. Description of the Prior Art
FIG. 1 is a circuit diagram showing one example of the conventional induction heating apparatus interesting for the present invention. This prior art is disclosed in the Japanese Patent Publication No. 36473/1983 published on Aug. 9, 1983.
An AC voltage from a low-frequency AC power supply 1 is converted into a DC voltage by rectifying circuit 3, and the DC voltage from this rectifying circuit 3 is applied to an inverter 5. The inverter 5 includes a capacitor 7 connected across input terminals of the DC voltage from the rectifying circuit 3, and this capacitor 7 acts as a moothing capacitor. A series connection of a heating coil 9 and a switching transistor 11 is connected with a smoothing capacitor 7. A resonance capacitor 13 and a fly-wheel diode 15 are connected between the collector and the emitter of the switching transistor 11. The resonance capacitor 13 constitutes and LC resonance circuit in cooperation with the heating coil 9.
To control turning on or off of the switching transistor 11, a control circuit 17 is provided, and five terminals 19a, 19b, 21a, 21b and 23 are connected to the control circuit 17. The DC voltage from the rectifying circuit 3 is taken into the control circuit 17 through the terminals 19a and 19b. On the other hand, a voltage of the point where the heating coil 9 and the switching transistor 11 are connected in series, that is, the collector voltage of the switching transistor 11 is taken into the control circuit 17 through the terminal 23. In the control circuit 17, the two voltages thus taken are compared with each other, and based on this comparison, a switching pulse for turning on or off the switching transistor 11 is outputted to the terminals 21a and 21b. More specifically, when the collector voltage from the terminal 23 becomes lower than the DC voltage from the terminal 19a, after a certain time set by a delay circuit (not illustrated), the switching transistor 11 is turned on.
In accordance with this prior art, timing of turning on of the switching transistor 11 is controlled based on the comparison between two voltages, and therefore an effect can be expected that a stable oscillation can be sustained even when a comparatively large variation in load takes place.
However, this prior art cited above still leaves the following problem to be solved. More specifically, in this prior art, the base current of the switching transistor is controlled based on the comparison of the DC power supply voltage with the collector voltage of the switching transistor, and therefore particularly when the oscillation frequency of the resonant circuit becomes high, switching loss of the switching transistor becomes large. To be further detailed, when the oscillation frequency is raised, fall of the collector voltage of the switching transistor becomes slow, while the magnitude of the DC voltage is not varied, and accordingly the timing when the collector voltage becomes smaller than the DC voltages becomes earlier. In other words, when the frequency is higher, the timing of turning on of the switching transistor becomes earlier compared with the case where the frequency is lower. At such an earlier timing, the collector voltage of the switching transistor does not fall enough, and accordingly the switching transistor is turned on in a state where the collector voltage of the switching transistor is comparatively large. Consequently, an inrush current which flows at an instant of turning on of the switching transistor becomes large, and such an inrush current produces a large switching loss. Thus, in the prior art as cited above, particularly when the oscillation frequency is high, the switching loss becomes large and thereby a reduction in efficiency becomes a problem.